Thermal Analysis and Investigation of NiO-Based Oxygen Carriers for Chemical-Looping Combustion

Licentiate thesis, 2009

Capture and storage of CO2 can be used to reduce greenhouse gas emissions from combustion of fossil fuels. Chemical-looping combustion is a two-step combustion process where CO2 is obtained in a separate stream, ready for compression and storage. The technology uses circulating oxygen carriers to transfer oxygen from an air reactor to a fuel reactor, thus avoiding an energy consuming gas separation unit. A thermal analysis of the process using a large number of possible oxygen carriers was performed by simulating chemical reactions. Based on the ability of the oxygen carriers to convert different gaseous fuels, stability in air and melting temperature some metal oxides based on Ni, Cu, Fe, Mn, Co, W and sulphates of Ba, Sr and Ca showed good thermodynamic properties and could be feasible as oxygen carriers. The promising systems were investigated further with respect to temperature changes in the fuel reactor as well as possible formation of carbon, sulphides and sulphates which may deactivate the oxygen carriers. Oxygen carriers of NiO, supported by NiAl2O4, were prepared and investigated experimentally with respect to parameters important for chemical-looping combustion. These oxygen carriers were based on commercially available raw materials in contrast to most of the previously tested oxygen carriers, which have been prepared from pure chemicals. Further, it was investigated if spray-drying, which is a production method suitable for large-scale particle preparation, can be used to produce high performing oxygen carriers instead of the small-scale freeze-granulation method. Generally, materials prepared from commercially available raw material showed high reactivity with methane and oxygen. Oxygen carriers prepared by spray-drying, displayed a remarkable similarity when compared to oxygen carriers prepared from the same starting material by freeze-granulation, both regarding physical properties and reactivity. Further, the up-scaling of the particle production did not present any difficulties. To reduce the risk of fragmentation and attrition of these oxygen carriers in a circulating chemical-looping combustion system, the strength of the oxygen carriers can be improved by an addition of Ca(OH)2, by increasing the sintering temperature or by extending the sintering time. Some unreacted CH4 was released at high degrees of oxygen carrier oxidation. This fraction of unconverted CH4 was fairly unchanged when the sintering temperature was increased while an extended sintering time resulted in a clearly improved conversion. Materials with MgO added during particle preparation or with MgAl2O4 as supporting agent resulted in considerably increased CH4 conversion.